Advanced method for WWER RPV embrittlement assessment

Abstract

The integrity assessment of reactor pressure vessels (RPV) is a main issue of NPP safe operation. Mechanical properties of the PV materials are degrading progressively under neutron irradiation reducing safety margins. To estimate the current state of embrittlement and to predict actual RPV safe life, it is necessary to develop improved radiation embrittlement understanding models. This is particularly true for the case of re-embrittlement after annealing, typical of several VVER-4000 RPVs. In the present work, on the base of analysis of available information, a semi-mechanistic model is offered. The model is based on the additive contribution of the three main known embrittlement mechanisms: matrix damage, precipitations and segregations which are simply described analytically since the understanding of the role P and Cu in enhancing irradiation embrittlement is rather well known. The model is used to explain and predict embrittlement and re-embrittlement after annealing of high P materials typical of certain VVER-440 RPVs. The proposed model. In this case, proves to have a very practical relevance, since it explains the peculiar re-embrittlement kinetics at low fluence after annealing. This is important for the correct interpretation of early surveillance data avoiding un-correct prediction for longer term embrittlement.